How the Node.js Event Loop Works
Node.js uses an event-driven architecture. JavaScript executes on the main thread while asynchronous work can be handled by the operating system or libuv. Once asynchronous operations complete, callbacks are scheduled for execution.
The event loop contains phases such as timers, pending callbacks, poll, check, and close callbacks. Understanding these phases helps explain the behavior of timers, sockets, and asynchronous I/O.
Single Thread Does Not Mean No Concurrency
Node.js executes JavaScript on a main thread, but asynchronous I/O and libuv worker threads allow many operations to progress concurrently.
Simplified Event Loop
Key Takeaways
- The event loop coordinates asynchronous callbacks.
- Non-blocking I/O allows a Node.js process to handle many concurrent connections.
- CPU-heavy JavaScript still blocks the event loop unless moved elsewhere.
Knowledge Check
What happens when CPU-heavy synchronous JavaScript runs on the event-loop thread?